Optical Add/Drop Multiplexer Has Variable Bandwidth

At the node of a fiber optic telecom system, it is often necessary to remove the data traveling in a particular wavelength channel and replace it with new data. A link between New York and Baltimore, for example, may contain information destined for Philadelphia (which is between the two cities). In Philadelphia, that information must be removed, and the now-empty channel can be used to transmit new information from Philadelphia to Baltimore. An optical add/drop multiplexer is the device used to drop data from and add it to a given channel.

As channel loads change and other circumstances occur, it can be desirable to reconfigure an optical add/ drop multiplexer, and microdisk resonators with waveguides activated by microelectromechanical systems (MEMS) have been investigated as a possible mechanism for these reconfigurable devices.

Recently, several of the researchers pursuing this approach demonstrated what they believe are the first microdisk resonators whose coupling bandwidth is adjustable. Their basic concept involves using a MEMS actuator to change the spacing between a microdisk resonator and the waveguides coupling light into and out of it (Figure 1).

Figure 1. In the decoupled condition (left), the waveguides are far enough from the microdisk that no energy is coupled into it. All the input energy is transmitted to the through port (dotted line), and none to the drop port (solid line). When the waveguides are brought closer to the resonator (center), energy is coupled into the resonator and then out the drop port. However, only frequencies resonant in the microdisk — that is, only wavelengths that will fit an integral number of times around its circumference — are coupled. When the waveguides are brought even closer to the microdisk (right), the maximum transmission to the drop port is not significantly changed, but its bandwidth increases. Images reprinted with permission of Optics Letters.An optical add/drop multiplexer with tunable bandwidth offers several potential advantages, including optical performance monitoring and dynamic bandwidth allocation. Other approaches to bandwidth tuning, such as mechanically stretching a fiber Bragg grating, have been investigated by other researchers, but these techniques are bulky and awkward compared with the MEMS-activated waveguides.

Figure 2. The researchers suspended the flexible waveguides above the microdisk and pulled them down toward the disk by applying an electrostatic force.For their demonstration, the researchers, who were from National Tsing Hua University in Hsinchu, Taiwan, and the University of California, Berkeley, fabricated a 20-μm-radius microdisk in silicon and a pair of 0.8-μm-wide deformable waveguides. They suspended the waveguides 1.0 μm above the edge of the microdisk (Figure 2). By applying an electrostatic force at the ends of the waveguides, they could move them downward, reducing the gap between them and the disk.

Figure 3. As the electrostatic force drew the waveguide closer to the microdisk, nearly all the input power was transferred from the through port to the drop port.Experimentally, they saw all the input power transmitted to the through port when they applied no voltage to the electrostatic actuators, and nearly all the input power transmitted to the drop port when they applied the maximum voltage (Figure 3). Between these extremes, the sum of power transmitted to both ports was less than the input power because a significant amount was lost in the microdisk. At either extreme, the power circulating in the disk was minimal, but between the extremes, the propagation loss in the disk could become a significant fraction of the total input power.

Figure 4. A 7-V variation in the voltage applied to the actuators changed the coupling bandwidth from 12 to 27 GHz, while not affecting the 20-dB extinction ratio.
Applying an equal voltage to the actuators for both waveguides, the researchers observed that the coupling bandwidth increased from 12 to 27 GHz while maintaining a 20-dB extinction ratio, as they adjusted the voltage from 23 to 27 V (Figure 4). Over the entire voltage range — 0 to 35 V — they observed the bandwidth vary from 3 to 53 GHz, albeit with a smaller extinction ratio